This invention relates to a process for the electrostatic purification of dust- and pollutant-containing exhaust gases in multiple-field precipitators, in which the exhaust gases are first subjected in a first stage in the direction of flow to an electrostatic purification under dry conditions in gas passages defined by platelike collecting electrodes and are subsequently passed in a second stage through one or more fields which are defined by liquid-wetted collecting electrodes, which define gas passages. The invention relates also to an apparatus for carrying out the process.
In known processes for electrostatic purification of dust- and pollutant-containing exhaust gases the latter are subjected to an electrostatic purification under dry conditions in a first processing stage and in a succeeding second processing stage are subjected to an electrostatic purification under wet conditions. British Patent Specification 988,350 describes an electric dedusting process in which a drying tower, one or more electric fields operating under dry conditions, and one or more electric fields operating under wet conditions are arranged in succession. The water is sprayed through nozzles into the wet field or fields and is drained as a slurry, which is concentrated in thickeners and is then injected by means of steam or compressed air into the drying tower, in which the evaporated liquid humidifies the hot drying gas so that a back corona discharge in the fields operating under dry conditions is prevented. From the article "Hybrid-type electrostatic precipitator" by Masuda, Air Pollut, Control Assoc. 1977, 27(3), 241-1 (Eng.) it is apparent that in such process acid components such as SO.sub.x, HF and HCL, are absorbed from the liquid which has been sprayed into the wet stage and together with the dust which is still collected in the wet stage enter a sump disposed in the wet stage. A disadvantage of that process resides in that the slide formed in the sump of the wet stage contains a relatively large amount of pollutants in addition to the dust and for this reason can be processed only with difficulty. A further disadvantage of that process resides in that the evaporated liquid which has been injected into the drying tower will moisten the dust-and pollutant-containing exhaust gas so that its dew point temperature will be increased. Because the gas temperature is decreased at the same time, the temperature in the electrostatic precipitator will decrease below the dew point temperature so that a corrosion caused by the acid components of the exhaust gas cannot be avoided.
U.S. Pat. No. 1,766,422 also describes the electrostatic purification of dust- and pollutant-containing exhaust gases in a process in which the exhaust gas laden with dust and pollutants is first subjected to an electrostatic purification under dry conditions and subsequently to an electrostatic purification under wet conditions. In that process that collecting electrodes of the wet electrostatic purification stage are wetted with a treating liquid. The electrostatic precipitator is operated at such a high gas velocity that the particles of the fine fraction will be collected in the dry electrostatic purification stage and those of the coarse fraction in the wet electrostatic purification stage. In that process, the sludge formed in the sump of the wet electrostatic purification stage and will contain a relatively large amount of pollutants in addition to the dust. An additional disadvantage of that process resides in that the exhaust gas is passed through the electrostatic precipitator at a relatively high gas velocity to ensure that the coarse particles of the dust contained in the exhaust gas can be collected in the wet electrostatic purification stage. As a result, the residence time of the exhaust gas in the wet electrostatic purification stage is not sufficient to ensure that the pollutants contained in the exhaust gas will be removed to such a degree that the limits prescribed in TA-Luft (German Regulation for Air Pollution Control) dated Feb. 27, 1986 , can be complied with.